The relationship between alteration of germinal center response and immune dysfunction in aging will be studied. In secondary lymphoid organs, germinal centers are sites of post-rearrangement B-cell repertoire modification by V(D)J hypermutation, receptor editing and affinity-driven clonal selection; germinal centers are also necessary for generation of the B-cell memory compartment. Furthermore, our recent work suggests that the germinal center reaction may recapitulate some aspects of primary lymphogenesis and plays a role in the induction and maintenance of self-tolerance in the periphery through selective apoptosis. With in situ and ex vivo approaches, I will test the hypothesis that altered germinal center response plays an important role in immunosenescence, including diminished immune response and heightened autoimmunity. Enzyme- and fluorochrome-labeled antibodies and lectins will be used to characterize and follow the responding lymphocyte populations, especially in reference to germinal centers. BrdUrd- and Tdt-labelling as well as antibodies to apoptosis-associated proteins will be used to study cell proliferation and programmed cell death. Cross-linking reagents for antigen-receptors, steroids and inhibitors for apoptosis will be applied to test the sensitivity and specificity of lymphocyte subpopulations to apoptotic signals. Microdissection of small groups or individual cells from histologic sections and FACS sorting of cell populations will be used in conjunction with various PCR amplification of V(D)J rearrangements, ds RSS breaks, and other gene segments of interest to study the somatic genetics of lymphocytes during the course of aging. These approaches will make possible to systemically study the effect of aging on germinal center response and its consequence.